Why most historians set the Nazi decision to exterminate Jews in Dec 1941 or early 1942, while the Einsatzgruppen had began the system killing of 1 million Jews by the end of 1941?

i think it is because of the "wannsee-konferenz" (20.1.1942) where the descicion to kill all jews systematically was presented, i.e. mass shootings and death-marshs(?) already started to take place in 1941. i am no historian, so maybe someone else will give a better, more in-depth answer but i hope this helps regardless!